Links:

“Fall Color in the Carlinas”:http://www.clemson.edu/extfor/publications/forlf25/ (Clemson University)

“Deciduous Forest”:http://www.blueplanetbiomes.org/deciduous_forest.htm (BluePlanetBiomes.org)

Author’s Notes:

Transcript of interview with Dr. John B. Shane, senior researcher and lecturer in forest physiology as well as program chair in the Forestry Program, University of Vermont.

SG:
The listener question is, when plants and trees lose their leaves, do they still go through the process of photosynthesis? And if they don’t how do they stay alive in the winter?

JS:
Well, it’s a complicated question, and the answer is yes and no. Some plants have been found to be able to photosynthesize with a little bit of chlorophyll in their barks. Certain species, where it has been documented that even without leaves, the plant is able to capture a certain amount of carbon through the process of photosynthesis in its bark. However, my guess is that’s a very small proportion of the total energy yield of the plant, and I’m not sure that every plant could do that.

The real answer to the question is imbedded in the part of it [that asks] how do they survive? They survive by the same way that any organism survives a period of winter, or a period when resources are lacking. They store energy that they captured during the growing season in the form of carbohydrates and sugars in their tissues, and they use that all winter long.

SG:
I’m trying to understand how they’re able to not be killed by extreme cold….

JS:
They harden their tissues off, and again, it’s a fairly complicated process, but it’s a series of biochemical steps that essentially protects those cells from freezing, and if we wanted to talk about the specific mechanisms that cells have evolved in plants to avoid frost damage, it’s a fairly complicated subject but it’s a process of trying to get rid of any extra water in the plant tissues to minimize that freezing process, and there’s production of compounds that actually sort of act almost like antifreeze, and there’s a whole bunch of membrane changes. It’s a long series of biochemical steps that various cells and plant tissues undergo that allows them to experience below-freezing temperatures without actually killing the cells themselves.

They essentially go dormant. But this is not confined simply to plants. There are a lot of animal tissues, for example, the pupae of certain insects, the larvae of certain insects, certain fish, and some amphibians, all which sort of go into this dormancy period where they are able to really experience extremes of temperature and not die.

SG:
Basically, their physiological processes almost grind to a halt?

JS:
Yes, they slow down, they essentially stop for all practical purposes, and the cells themselves are sort of in this dormant state, and also protected against freezing by all sorts of biochemical paths that they’ve gone through in fall.

SG:
There are different techniques you have described, do you see all of these in the winter trees, or do some trees use some processes and others don’t?

JS:
I’m not sure about that. I think you can see the general pattern in any tree that has to undergo an extreme winter. The reason why we don’t have some tree species here in Vermont is at least partly related to the fact that they aren’t capable of becoming sufficiently winter-hardy to withstand the cold. Even though they might be the same processes, they don’t undergo them to the same extent. Or they don’t undergo them at the correct time. So we might have a plant species that’s capable of being plenty hardy in January but in the process of getting to that level of hardiness, it didn’t get there fast enough and so was killed by a cold spell in October.

SG:
Even for the same species of tree? There would be geographical variations…

JS:
… like geographical races.

SG:
In terms of actually transporting water, they completely halt that?

JS:
That’s an unfortunate problem. Deciduous trees that lose their leaves essentially stop losing water because most of the water pathways [are] through the leaves, but there’s some water loss through the twigs and the stems, but it’s probably not so large [to be] a major problem for trees that lose their leaves. But for conifers, species that retain their needles, they are capable of losing water. In fact they do lose water in the winter and if their stem tissues or root tissues are frozen, or if the ground itself is frozen, they can desiccate fairly dramatically, and that’s one of the ways that a conifer tree will die in the winter.

SG:
Besides balancing the water in the plant during winter, what are some of the other stresses that plants are susceptible to?

JS:
In winter?

SG:
Yes

JS:
The major stresses in winter really are related to a couple of things. Energy relations, the capture of carbon and the use of that as food, water relations, as we talked about, and the whole idea of being able to tolerate those extreme temperatures. So really, what we’re talking about is the stresses that are incident in winter more than any other [season]…. the plant is consuming food to stay alive, but is probably producing little or none. And that balance has to be maintained.

The following person was interviewed for today’s program. Our thanks to:

Dr. John B. Shane
Lecturer and Senior Researcher
Department of Forestry
University of Vermont